Add new Lint/RegexpInCondition cop

[pocke]

Problem

If a regexp literal is in condition, the regexp matches $_ implicitly.
For example:

# test.rb
$_ = 'foo'
if /foo/
  p 1
end

$_ = 'bar'
if /foo/
  p 2
end

# It outputs 1, but it does not output 2.
$ ruby test.rb
1

The behaviour is confusing. I guess the feature is designed for one-liner. So, we should not use the feature in production code.

Solution

Add this cop 😄

Lint/LiteralInCondition?

This cop is very similar to Lint/LiteralInCondition cop. But this should be a different cop from Lint/LiteralInCondition.
There are two reasons.

First, these cops target different issues. Lint/LiteralInCondition targets the issue that a condition is evaluated as true. But Lint/RegexpInCondition targets the issue that a regexp literal matches $_ implicitly.

Second, these cops have different implementation.

Note

Currently, this cop does not work on ! and not.
For example:

# This code has the problem, but the cop does not add offence for the code.
!/re/

It is a parser gem's bug, and the bug is already fixed in master.
whitequark/parser#376

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[Drenmi]

Perhaps the message should be more specific, since e.g. if /foo/ =~ str is also a regexp literal in a condition. Perhaps the cop name should even be RegexpCondition? 🤔

[bbatsov]

Guess @pocke just followed the (bad) naming of the other cop. Maybe it should have been named LiteralAsCondition or LiteralCondition.

[Drenmi]

Ah. yes.

[bbatsov]

Let's improve the cop name and their messages subsequently.